The invention relates generally to steam turbines and more specifically to steam turbine exhaust arrangements.
In the discharge of exhaust steam from an axial flow turbine, for example discharge of this exhaust steam to a condenser, it is desirable to provide as smooth a flow of steam as possible and to minimize energy losses from accumulation of vortices and turbulences and non-uniformity in such flow. Usually the exhaust from the turbine is directed into an exhaust hood and from there to through a discharge opening in the hood in a direction essentially normal to the axis of the turbine into a condenser. It is desirable to achieve a smooth transition from axial flow at the exhaust of the turbine to radial flow in the exhaust hood and thence a smooth flow at the discharge opening of this hood into the condenser.
In the constructing of an effective exhaust hood for use with such an axial flow turbine it is desirable to avoid acceleration losses within any guide means employed therein and to achieve a relatively uniform flow distribution at the discharge opening of the exhaust hood for the most efficient conversion of energy in the turbine and effective supplying of exhaust steam to the condenser to which it is connected.
It is also desirable to achieve optimum efficiency at the last stage buckets of the turbine prior to exhaust from the turbine by achieving a relatively uniform circumferential and radial pressure distribution in the exit plane of the last stage buckets. Usually, attempts have been made to accomplish these results while employing a hood having as short an axial length as possible, so as to limit the axial size of the turbine train.
The prior art has employed, in the exhaust duct connected to the turbine, vanes, which have smoothly curved surfaces for effectively changing the axial flow of the steam from the turbine to the generally radial flow. For example of such an arrangement for converting the axial flow of the exhaust from the turbine to radial flow is shown in U.S. Pat. No. 3,552,877 by Christ et al. Further developments in prior art exhaust hoods for axial flow turbines, such as U.S. Pat. No. 4,013,378 by Herzog, have incorporated multiple sets of vanes for further smoothing flow. The exhaust hood includes a first set of guide vanes arranged in an exhaust duct connected to the turbine adjacent the last stage buckets thereof. These vanes are curved to provide a relatively smooth transition of steam flow from an axial direction to a generally radial direction. A guide ring circumferentially surrounds the first set of guide vanes and a plurality of secondary vanes are circumferentially spaced around this guide ring. Steam, which is discharged radially from the first set of vanes to the secondary vanes, is directed by the secondary vanes to the discharge opening of the exhaust hood. The secondary vanes are substantially equally spaced around the guide ring and are curved at different angles to effect different angles of discharge of steam from these vanes. The angles of discharge are chosen so as to direct the steam toward the discharge opening of the exhaust hood in a manner achieving substantially uniform flow distribution across the exit plane of the last stage buckets and across the plane of the discharge opening. However, while such vanes may be optimized for one set of flow conditions, they may operate with significantly less effectiveness at other flows.
Diffusers, for example, are commonly employed in steam turbines. Effective diffusers can improve turbine efficiency and output. Unfortunately, the complicated flow patterns existing in such turbines as well as the design problems caused by space limitations make fully effective diffusers almost impossible to design. A frequent result is flow separation that fully or partially destroys the ability of the diffuser to raise the static pressure as the steam velocity is reduced by increasing the flow area. For downward exhaust hoods used with axial steam turbines the loss from the diffuser discharge to the exhaust hood discharge varies from top to bottom. At the top, much of the flow must be turned 180 degrees to place it over the diffuser and inner casing, then turned downward. Pressure at the top is thus higher than at the sides, which are in turn higher than at the bottom.
Adding further complication to the function of exhaust hoods is a problem of exhausting to separate condensers from opposing turbine sections in a dual flow steam turbine, such as a dual flow, low-pressure steam turbine. Multiple pressure condensers are commonly used and improve the heat rate for two basic reasons. They provide a lower average back pressure and the condensate leaving the condenser has a higher temperature than single pressure condensers. Back pressure of multi-pressure units is lower because the heat rejection per unit length of condenser is more uniform. Thermodynamically this means that heat is transferred at a lower average temperature difference, that is, more efficiently. A double flow steam turbine with multiple flow paths to condensers is known (Nishioka, U.S. Pat. No. 4,306,418).
Opposing sections of dual axial flow steam turbines traditionally exhaust into a common exhaust hood that surrounds the opposing sections and then exhaust into a common condenser. In order to exhaust to separate condensers of separate sections of a multi-section condenser, it is known to utilize baffles that divides the exhaust hood for each of a first turbine section and a second turbine section. The baffling may further divide the condenser into separate sections, each separate section of the condenser in fluid communication with one of the divided sections of the exhaust hood. Thus the opposing turbine sections may be exhausted into separate condenser sections, with different operating pressures. (See Silvestri et al., U.S. Pat. No. 4,557,113).
It is further known (Silvestri, U.S. Pat. No. 5,174,120) to provide a vertical divider plate in the exhaust flow from an outlet in each of opposing sections of a double flow steam turbine and directing the divided flow to separate condensers. More specifically, the vertical divider plate separates the flow from the annulus of the turbine outlet (at a respective end of the turbine section) flowing between an inner flow guide and outer flow guide. A further vertical divider plate(s) separates the exhaust hood vertically along an axial direction. The vertically divided exhaust hood may then be placed in communication with condensers of separate pressure, allowing a lateral separation of the exhaust from the turbine section. The laterally separated exhaust may then be directed to specific condensers.
FIG. 1 illustrates a perspective partial cutaway of a double flow steam turbine a portion of a steam turbine. FIG. 2 illustrates a portion of the double flow steam turbine including an exhaust flow path. The steam turbine, generally designated 10, includes a rotor 12 mounting a plurality of turbine buckets 14. An inner casing 16 is also illustrated mounting a plurality of diaphragms 18. A centrally disposed generally radial steam inlet 20 applies steam to each of the turbine buckets and stator blades on opposite axial sides of the turbine to drive the rotor. The stator vanes of the diaphragms 18 and the axially adjacent buckets 14 form the various stages of the turbine forming a flow path and it will be appreciated that the steam is exhausted from the final stage of the turbine for flow into a condenser not shown.
Also illustrated is an outer exhaust hood 22, which surrounds and supports the inner casing of the turbine as well as other parts such as the bearings. The turbine includes steam guides 24 for guiding the steam exhausting from the turbine into an outlet 26 for flow to one or more condensers. With the use of an exhaust hood supporting the turbine, bearings and ancillary parts, the exhaust steam path is tortuous and subject to pressure losses with consequent reduction in performance and efficiency. A plurality of support structures may be provided within the exhaust hood 22 to brace the exhaust hood and to assist in guiding the steam exhaust flow. An exemplary support structure 30 is situated to receive and direct the steam exhaust flow 35 from the steam turbine 10. The diffusion of the steam is restricted to the volume in the exhaust hood 22.
The traditional exhaust hood arrangements described above, with vertical dividers, addresses the lateral separation of the exhaust from the turbine outlet. However, the traditional exhaust hood arrangement is not conducive to providing vertical division of the exhaust flow from the turbine outlet. Accordingly, it may be advantageous to provide an exhaust arrangement that vertically separates the flow from the upper and lower half of the turbine outlet exhaust annulus.